In recent years, from the viewpoint of environmental protection, various electric devices have generally been controlled by inverter circuits for promoting energy saving and highly efficient driving. Particularly, in car industries, since a hybrid electric vehicle (hereinafter, HEV) that runs on an electric motor and an engine has appeared on the market, car manufacturers have been promoting eco-friendly technology development focused on energy saving and high efficiency.
An HEV motor works in a high range of service voltage as high as several hundred volts. Considering above, manufacturers focus on a metallized film capacitor having preferable electric characteristics—high withstanding voltage and low loss—as being suitable for the HEV motor. Besides, in response to demands of the market on maintenance-free components, the use of the metallized film capacitor is increasing because of its extremely long life.
A metallized film capacitor for HEVs is demanded strongly to have an increased withstanding voltage, an increase allowable current, and an increased capacity. To meet the demands, an improved case mold type capacitor has been developed and into actual use. Such a case mold type capacitor has a structure formed in a manner that a plurality of metallized film capacitors connected in parallel by bus bars are accommodated in a case and then resin-molded with molding resin.
FIG. 12A is an exploded perspective view before resin molding of a conventional case mold type capacitor. FIG. 12B is a perspective view of the conventional case mold type capacitor as a complete product. FIG. 13 is a sectional view taken along the line 13-13 of FIG. 12B. Metallized film capacitor element (hereinafter, element) 20 is structured by winding a pair of metallized films and forming metal-sprayed electrodes by zinc spraying on both surfaces of the wound body of the films. The metallized film is formed in a manner that a metal deposition electrode is disposed on either or both of the surfaces of a dielectric film made of polypropylene. The metal deposition electrodes are wound so as to be opposite each other via the dielectric film. On the both ends of the wound body, a pair of metal-sprayed electrodes are disposed as a pair of lead-out electrodes. One is P-pole electrode and the other is N-pole electrode.
P-pole bus bar 21 is connected to the P-pole electrodes each of which is formed on one end surface of each of elements 20 disposed in close contact with each other. P-pole bus bar 21 has P-pole terminal section 21A on one end for external connection. P-pole terminal section 21A is led upward elements 20 and exposed out of case 23 that will be described later.
Like P-pole bus bar 21, N-pole bus bar 22 is connected to the N-pole electrodes each of which is formed on the other end surface of each of elements 20 disposed in close contact with each other. Similarly, N-pole bus bar 22 has N-pole terminal section 22A on one end for external connection. N-pole terminal section 22A is led upward elements 20 and exposed out of case 23 that will be described later. In this way, P-pole bus bar 21 and N-pole bus bar 22 establish parallel connection between a plurality of elements 20.
Case 23 is made of resin and is filled with molding resin 24. A plurality of elements 20 connected in parallel by P-pole bus bar 21 and N-pole bus bar 22 is accommodated in case 23 and then resin-molded with molding resin 24. Such structured case mold type capacitor is, for example, disclosed in patent document 1.
As described above, in the conventional case mold type capacitor, elements 20 are molded in case 23 with molding resin 24. The structure enhances mechanical strength, heat resistance, and moisture resistance.
However, when used for an HEV in the car industry, demanded conditions are high. For example, during a humidity test or a thermal cycle test, molding resin 24 of the conventional case mold type capacitor may fail in a complete block-off of moisture. Besides, if a crack occurs in molding resin 24 during the thermal cycle process, elements 20 can deteriorate due to moisture entry through the crack of molding resin 24.    Patent document 1: Japanese Unexamined Patent Publication No. 2004-146724